Quantitative Scanning Microwave Microscopy for Transfer Characteristics of GaN High-Electron-Mobility Transistors

This article demonstrates the feasibility in using a scanning microwave microscope (SMM) to probe the transfer characteristics of an ungated GaN high-electron-mobility transistor (HEMT). To guide the experiment and to interpret the result, an equivalent circuit is proposed to model the probe-sample near-field interaction, and the model is validated by simulation and experimentation. In the experiment, the SMM probe with a dc bias voltage acts as a surrogate to locally modulate the 2-D electron gas (2DEG) at the GaN heterojunction. Because the present SMM is most sensitive to a 2DEG sheet resistance <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{\text {SH}}$ </tex-math></inline-formula> between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{4}~\Omega/\square $ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{6}~\Omega/\square $ </tex-math></inline-formula>, the unbiased <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{\text {SH}}$ </tex-math></inline-formula> is determined to be (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3~\pm ~3$ </tex-math></inline-formula>) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10^{3}~\Omega/\square $ </tex-math></inline-formula>, in contrast to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 450~\Omega/\square $ </tex-math></inline-formula> determined by Hall measurements. However, with the bias decreasing from 0 to −8 V, the 2DEG is depleted and its resistance is increased to (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5~\pm ~2$ </tex-math></inline-formula>) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10^{5}~\Omega/\square $ </tex-math></inline-formula> with an on/off ratio of 160, a peak transconductance around −5 V, and a threshold voltage of −6 V. These results agree with the dc-measured current-voltage characteristics on a gated HEMT after its fabrication is completed. This shows that the SMM could be a powerful tool for in-process monitoring and material/device correlation.